ee 330 lecture 31 current source biasing current sources and mirrors

EE 330Lecture 31

• Current Source Biasing• Current Sources and Mirrors

B

E

C

VDD

Vin

RC

Vout

RE

VEE

B

E

C

VSS

VDD

Vin

RE

VoutB

E

C

VDD

Vin

RC

Vout

VEE

AV

C

E

R-R

Rin

Rout

π Er +βR

CRC R

m Cg R

-1mg

B

E

C

VBB

VDD

Vin

RC

Vout

m

m E

g

g +g

π Er +βR

m C-g R

πr

CR

tE

CQ

Vβ R

I

CQ C

t

I R

VDQ C

EB

2I R

V

DQ

EB

2I

VCQ

t

I

V

CQ E

CQ E t

I R

I R +VDQ E

DQ E EB

2I R

2I R +V

tE

CQ

Vβ R

I

t

CQ

βV

I

DQ C

EB

2I R

V CQ C

t

I R

V

-1mg

DQ

EB

2I

VCQ

t

I

V

MOS BJT MOS MOS MOSBJT BJT BJT

CE/CS CC/CD CB/CG CEwRE/CSwRS

Basic Amplifier Gain Table

Can use these equations only when circuit is EXACTLY like that shown above !!

Review from Last Lecture

B

E

C

VDD

Vin

RC

Vout

RE

VEE

B

E

C

VSS

VDD

Vin

RE

Vout

B

E

C

VDD

Vin

RC

Vout

VEE

B

E

C

VBB

VDD

Vin

RC

Vout

CE/CS

CC/CD

CB/CG

CEwRE/CSwRS

Basic Amplifier Characteristics Summary

•Reasonably accurate but somewhat small gain (resistor ratio) •High input impedance •Moderate output impedance•Used when more accurate gain is required

•Large noninverting gain•Low input impedance •Moderate (or high) output impedance•Used more as current amplifier or, in conjunction with CD/CS to form two-stage cascode

•Large noninverting gain•Low input impedance •Moderate (or high) output impedance•Used more as current amplifier or, in conjunction with CD/CS to form two-stage cascode

•Gain very close to +1 (little less)•High input impedance for BJT (high for MOS)•Low output impedance•Widely used as a buffer


High-gain BJT amplifierm

V m C0 C

-gA -g R

g G

B

E

C

VDD

Vin

RC

Vout

VEE

To make the gain large, it appears that all one needs to do is make RC large !

CQ CV m C

t

-I RA -g R

V

But Vt is fixed at approx 25mV and for good signal swing, ICQRC<(VDD-VEE)/2

DD EEV

t

V VA

2V

If VDD-VEE=5V,

100V5V

A2 25mV

Gain is practically limited with this supply voltage to around 100


High-gain MOS amplifierm

V m D0 D

-gA -g R

g G

G

S

D

VDD

Vin

RD

Vout

VSS

To make the gain large, it appears that all one needs to do is make RD large !

DQ DV m D

EB

-2I RA -g R

V

But VEB is practically limited to around 100mV and for good signal swing, IDQRD<(VDD-VSS)/2

DD SSV

EB

V VA

V

If VDD -VSS=5V and VEB=100mV,

50V5V

A100mV

Gain is practically limited with this supply voltage to around 100

Are these fundamental limits on the gain of the BJT and MOS Amplifiers?


High-gain amplifier

VIN

VOUT

Q1

VDD

VEE

IB

VIN

VOUT

Q1

mV

-gA

0

gmVBEVBE

iB

gπVIN

VOUT

This gain is very large !

Too good to be true !

Need better model of MOS device!


High-gain amplifier

VIN

VOUT

Q1

VDD

VEE

IB

VIN

VOUT

Q1

mV

0

-gA

g

gmVBEVBE

iB

gπ g0VIN

VOUT

CQ AFV

t CQ AF t

-I VA -

V I /V V

8000AFV

t

V 200VA -

V 25mV

This gain is very large (but realistic) !

But how can we make a current source?


High-gain amplifier

VIN

VOUT

Q1

VDD

VEE

IB

8000VA

How can we build the ideal current source?

What is the small-signal model of an actual current source?

Current Sources/Mirrors

Q0

VCC

R

Q1AE0 AE1

Load

I1I0

CC0

V -0.6VI

R

If the base currents are neglected


Q0

VCC

R

Q1AE0 AE1

Load

I1I0

CC0

V -0.6VI

R

If the base currents are neglected

BE0

t

VV

0 S E0I =J A e

BE1

t

VV

1 S E1I =J A e

since VBE1=VBE2

1E1

0E0

AI I

A

Behaves as a current source !

Actually termed a “sink” current since coming out of load


Q0

VCC

R

Q1AE0 AE1

I1I0 I1

• Multiple Outputs Possible• Can be built at sourcing or sinking currents• Also useful as a current amplifier• MOS counterparts work very well and are not plagued by base current

Current Sink


Q0Q1

AE1

I1

I0

Q2

I2Qn

In

AE2 AEnAE0

Multiple-Output Bipolar Current Sink

Ekk 0

E0

AI = I

A


Multiple-Output Bipolar Current Source

Ekk 0

E0

AI = I

A

VDD

Q0 Q1 Q2 Qn

AE0 AE1 AE2 AEn

I1 I2 InI0


Multiple-Output Bipolar Current Source and Sink nkI =?

Q0Qn1

AE0 AEn1

In0I0

Qn2

In1

Qnn

Inn

AEn2

VDD

Qp0 Qp1 Qp2 Qpn

AEp0 AEp1 AEp2 AEpn

Ip1 Ip2 Ipn

AEnn

pkI =?


Multiple-Output Bipolar Current Source and Sink

Enknk 0

E0

AI = I

A

Q0Qn1

AE0 AEn1

In0I0

Qn2

In1

Qnn

Inn

AEn2

VDD

Qp0 Qp1 Qp2 Qpn

AEp0 AEp1 AEp2 AEpn

Ip1 Ip2 Ipn

AEnn

EpkEn1pk 0

E0 Ep0

AAI = I

A A


Q0 Q1

AE1

IoutIin

AE0

• Termed a “current mirror”• Output current linearly dependent on Iin

• Serves as a current amplifier• Widely used circuit

npn Current Mirror

E1out in

E0

AI = I

A


npn current mirror amplifier out=?i

Q0 Q1

AE0 AE1

iinIBS MIBSiout

E1

E0

AM=

A


Amplifiers both positive and negative currents

npn current mirror amplifier

E1out in

E0

A=

A

i i

Q0 Q1

AE0 AE1

iinIBS MIBSiout

E1

E0

AM=

A


n-channel Current Mirror

outI =?

M0 M1

W0,L0

Iout

Iin

W1,L1Q0Q1

AE1

IOU T

I0

AE0

npn Current Mirror


n-channel Current Mirror

2 1out in

1 2

W LI = I

W L

M0 M1

W0,L0

Iout

Iin

W1,L1

2OX 1in GS1 T1

2

μC WI = V -V

2L

2OX 2out GS2 T2

2

μ C WI = V -V

2L

If process parameters are matched, it follows that

Current mirror gain can be accurately controlled

Layout is important to get accurate gain (for both MOS and BJT)

Layout of Current MirrorsExample with M = 2

2 1

1 2

W LM=

W L

Standard layout

Gate area after fabrication depicted

2 2

2 22 1

1 2

W W L LM=

W W L L

2 22

2 21 1

1 1

2W W L LM=

W W L L


2 1

1 2

W LM=

W L

Standard layout

Better Layout

4 22

2 21 1

1 1

2W W L LM=

W W L L

2 22

2 21 1

1 1

2W W L LM=

W W L L


2 1

1 2

W LM=

W L

Standard layout

Better Layout

4 22

2 21 1

1 1

2W W L LM=

W W L L

Even Better Layout

4 22

2 21 1

1 1

2W W L LM=

W W L L

This is termed a common-centroid layout


n-channel current mirror current amplifier

2 1out in

1 2

W L=

W L

i i

M1 M2

W1,L1

iinIBS MIBSiout

2 1

1 2

W LM=

W L

W2,L2

Amplifiers both positive and negative currents


0kk 0

0 k

LWI = I

W L

M0 M1

W0,L0

I1

I0

M2

I2Mn

In

W1,L1 W2,L2 Wn,Ln

VDD

I1 I2 InI0

M1 M2 Mn

W1,L1 Wn,LnW2,L2

M0

W0,L0

multiple output n-channel current sink array

multiple output p-channel current source array

High-gain amplifier

VIN

VOUT

Q1

VDD

VEE

IB

8000VA

How can we build the current source?

What is the small-signal model of an actual current source?

Basic Current Sources and Sinks

VXX

IX

IX XX

t

V

VX S EI = J A e

IX

VCC

R

CCX

V -0.6VI

R

IX

Basic Bipolar Current Sinks Basic Bipolar Current Sources

IX

VCC

VYY IX

VCC

IX

VCC

IX

VCC

R

Very practical methods for biasing the BJTs can be usedCurrent Mirrors often used for generating sourcing and sinking currents

Basic Current Sources and SinksSmall-signal Model of BJT Current Sinks and Sources

VXX

IX

gmVBEVBE

iB

gπ g0 g0

Small-signal model of all other BJT Sinks and Sources are the same

Basic Current Sources and SinksSmall-signal Model of BJT Current Sinks and Sources

gmVGSVGSg0 g0

Small-signal model of all other MOS Sinks and Sources are the same

High-gain amplifier

VIN

VOUT

Q1

VDD

VEE

IB

VIN

VOUT

Q1 mV

0

-gA

g

VINVOUT

Q1

Q2

VIN

VOUT

Q1

VEE

VCC

VYY gm1VB

E1VBE1

iB

gπ1 g01 g02VIN

VOUT

m1 m1V

01 02 01

-g -gA

g g 2g

High-gain amplifier

VIN

VOUT

Q1

VDD

VEE

IB

mV

0

-gA

g

VIN

VOUT

Q1

VEE

VCC

VYY

m1V

01

-gA

2g

•Nonideal current source decreased the gain by a factor of 2

•But the voltage gain is still quite large

Can the gain be made even larger?

ee 330 lecture 31 current source biasing current sources and mirrors

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